A male or female who interferes with a female's attempts to get some sexual action. Similar to a cock blocker, except it's a vagina that's being blocked from action, hence the term "box."
"I would totally have picked up that guy if you douche bags hadn't been box blocking me all night."
"I like going out with the guys to watch the game, but I never get laid those nights since they're such box blockers."
When you are waiting to return your rental at a Redbox kiosk and the person in front of you is searching through every movie to find the perfect one.
When I returned my copy of The Dark Knight Rises at the Redbox, I was box-blocked for nearly 15 minutes while some jerk wearing pajama pants finally decided on The Best Exotic Marigold Hotel.